Electronic messages are often communicated using TDMA techniques. Within a transmitted TDMA signal, diverse time slots are allotted to diverse channels or purposes. In other words, a single TDMA signal communicates many independent messages by interleaving the messages in time. Typically, each of the independent messages is assigned its own time slot within a frame of time. The frames are often of constant duration, and a message's time slot assignment often remains consistent from frame to frame. Synchronization refers to the process of causing two stations, for example a base unit and a mobile unit, to operate with a common time base so that each has a common understanding of time slot and framing definitions.
The synchronization process often has multiple stages wherein initial stages are dedicated to establishing a communication link between two stations and later stages utilize the communication link to refine the synchronization definition so that messages may be communicated as efficiently as possible. The present invention is concerned with the initial stages of synchronization. In these initial stages, a slave station, often a mobile unit, synchronizes itself to a time and/or frequency base of a master station, often a base unit. When multiple master stations are present, the initial synchronization stage selects the master station with which to synchronize. After the slave unit has acquired synchronization, the slave station then transmits an initial message to the selected master station.
Slave stations in conventional communication systems utilize well known filtering and phase lock loop circuits to acquire an acquisition signal broadcast by a master station. When the conventional slave station has extensive a priori knowledge of the characteristics of an acquisition channel over which the acquisition signal is broadcast, the initial synchronization can occur quickly. For example, when the slave station knows the acquisition channel's frequency with precision, initial synchronization occurs quickly. When a continuous wave (CW) acquisition channel is used and no time slot information needs to be resolved, initial synchronization occurs quickly. Likewise, when interference between multiple acquisition channels need not be considered, initial synchronization occurs quickly.
However, in situations where a TDMA acquisition channel is utilized and where the acquisition channel's frequency is not known or when multiple TDMA channels at different frequencies are present, conventional synchronization techniques fail to achieve a quick synchronization. This situation occurs in connection with a satellite-based cellular TDMA communication system in which satellites act as master stations and travel at speeds of up to 26,000 Km/Hr relative to the earth. In such a cellular system, extensive Doppler frequency uncertainty results from the fast satellite movement. Moreover, the passing of calls or communication channels between satellites without losing data, along with a desire to maximize user convenience, suggest a need for rapid initial synchronization.